7 

Maps Showing the Change in Modern Sediment Thickness on 

the Inner Continental Shelf Offshore of Fire Island, New York, 

Between 1996–97 and 2011

Open-File Report 2014–1238

U.S. Department of the Interior

U.S. Geological Survey

Maps Showing the Change in Modern Sediment Thickness 

on the Inner Continental Shelf Offshore of Fire Island, New 

York, Between 1996–97 and 2011

By William C. Schwab, Wayne E. Baldwin, and Jane F. Denny

Open-File Report 2014–1238

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Schwab, W.C., Baldwin, W.E., and Denny, J.F., 2014, Maps showing the change in modern sediment thickness on 

the inner continental shelf offshore of Fire Island, New York, Between 1996–97 and 2011: U.S. Geological Survey 

Open-File Report 2014–1238, 8 p., http://dx.doi.org/10.3133/ofr20141238.

ISSN 2331-1258 (online)
Acknowledgments

We thank the captain and crew of the motor vessel Scarlett Isabella for their skill and cooperation at sea. Brian 

Johnson, from Coastal Carolina University, and Emile Bergeron, Eric Moore, Charles Worley, and Barry Irwin, all 

with the U.S. Geological Survey, provided technical support at sea. Andrea Toran was extremely helpful in the 

publication of this report. We thank Dave Foster and Larry Poppe for their reviews of this manuscript.

iii
Contents

Acknowledgments.......................................................................................................................................................iii

Abstract ...................................................................................................................................................................... 1

Introduction................................................................................................................................................................. 1

Regional Geologic Setting .......................................................................................................................................... 1

Data Collection and Processing.................................................................................................................................. 2

Mapping Results......................................................................................................................................................... 3

References Cited........................................................................................................................................................ 4

Figures

1. Map showing the survey area (shaded in dark gray) offshore of Fire Island, New York, 2011.. .................... 5

2. High-resolution chirp seismic-reflection profile and interpretation illustrating stratigraphic features and 

geometries discussed in the text.................................................................................................................... 6

3. A, Map showing the change in modern sediment thickness greater than 0.5 meter (m) between isopachs 

interpreted from 1996–97 (Foster and others, 1999) and 2011 (Schwab, Denny, and Baldwin, 2014) 

seismic-reflection data, overlain on the 2011 modern sediment thickness isopach (Schwab, Denny, and 

Baldwin, 2014), offshore of Fire Island, New York. Blue line shows seaward extent (toe) of the shoreface 

(Schwab, Baldwin, Denny, and others, 2014). Regional bathymetric contours are in meters below the North 

American Vertical Datum of 1988 (NAVD 88). Figure modified from Schwab, Baldwin, Denny, and others 

(2014). B, Enlargement of area shown in A. .................................................................................................. 7

4. Maps showing the 1- and 3-meter (m) contours of sediment thickness from the 1996–97 and 2011 surveys 

offshore of Fire Island, New York, of the A, shoreface-attached sand ridges and B, area of accretion on the 

lower shoreface overlain on the 2011 modern sand thickness (Schwab, Denny, and Baldwin, 2014)........... 8 

iv
Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

Velocity

meter per second (m/s) 3.281 foot per second (ft/s) 

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the World Geodetic System 1984 (WGS 84).

Depth, as used in this report, refers to distance below the vertical datum.

v
Maps Showing the Change in Modern Sediment Thickness 

on the Inner Continental Shelf Offshore of Fire Island, New 

York, Between 1996–97 and 2011

By William C. Schwab, Wayne E. Baldwin, and Jane F. Denny

Abstract

The U.S. Geological Survey mapped approximately 336 square kilometers of the lower 

shoreface and inner continental shelf offshore of Fire Island, New York, in 1996 and 1997, using highresolution sidescan-sonar and seismic-reflection systems, and again in 2011, using interferometric sonar 

and high-resolution chirp seismic-reflection systems. This report presents a comparison of sediment 

thickness and distribution as mapped during these two investigations. These spatial data support 

research on the Quaternary evolution of the Fire Island coastal system and provide baseline information 

for research on coastal processes along southern Long Island.

Introduction

In 1996 and 1997, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of 

Engineers, conducted a program to produce geologic framework maps of the nearshore areas south of 

Long Island, New York (Schwab and others, 1997; Foster and others, 1999; Schwab, Thieler, Allen, and 

others, 2000). Mapping methods included high-resolution sidescan-sonar and seismic-reflection 

techniques. The goal of this investigation was to determine regional-scale availability of sand as a 

resource for future beach nourishment programs and to investigate the role that inner continental shelf 

morphology and geologic framework have in the evolution of the coastal region of southern Long 

Island.

In 2011, a high-resolution marine geophysical survey of the lower shoreface and inner 

continental shelf was conducted offshore of Fire Island, New York (Schwab, Denny, and Baldwin,

2014). This report presents maps of the change in modern sediment thickness and distribution detected 

between the 1996–97 and 2011 mapping investigations. The results suggest that the nearshore/shoreface 

sedimentary deposit has gained sediment at the expense of continued erosion of the marine transgressive 

surface. This report adds to a scientific foundation used to manage coastal systems and assess 

environmental changes caused by natural processes and human activities.

Regional Geologic Setting

Long Island marks the southern terminus of the Wisconsinan Laurentide glacial advance in the 

eastern part of North America (Stone and Borns, 1986). The coast from Southampton to Montauk Point 

is a headland region where the Ronkonkoma moraine (fig. 1) and associated glacial outwash sediment 

1
are eroded directly by wave action (Williams, 1976). The south shore of Long Island west of 

Southampton consists of reworked outwash and includes shallow back-barrier bays, marshes, and lowrelief, sandy barrier islands (Leatherman and Allen, 1985). Located within this barrier-island system is 

Fire Island, a 0.5- to 1.0-kilometer (km)-wide, 50-km-long barrier island that is bound by two tidal 

inlets, Moriches Inlet to the east and Fire Island Inlet to the west (fig. 1).

Pleistocene glaciofluvial outwash deposits are exposed over much of the inner continental shelf 

south of Fire Island. The upper surface of the Pleistocene deposits was incised by a series of 

paleochannels that were subsequently filled with a transgressive sequence of glaciofluvial sediment, in 

places capped by lower Holocene muddy estuarine sediment (fig. 2). The modern sand deposit is 

derived from erosion of these Pleistocene glaciofluvial and lower Holocene fluvial channel-fill deposits 

exposed on the inner continental shelf by oceanographic processes during the Holocene marine 

transgression. The distribution of the modern sand deposit is discontinuous and variably thick, and it lies 

unconformably atop the Pleistocene and lower Holocene deposits (figs. 2 and 3). This unconformity is 

interpreted to be the Holocene transgressive surface. For a full review of the major inner continental 

shelf sedimentary sequences offshore of New York, see Schwab, Baldwin, Denny and others (2014) and 

Schwab, Denny, and Baldwin (2014) and references therein.

Data Collection and Processing

The area offshore of Fire Island was surveyed by using high-resolution sidescan-sonar, seismicreflection, bathymetric, and sediment-sampling techniques in May 1996 aboard the research vessel

Seaward Explorer, and in May 1997 and September to October 1997 aboard the research vessel Diane 

G. Details of the sidescan-sonar and fathometer acquisition and processing are described by Schwab, 

Thieler, Denny, and others (2000), and details of the seismic-reflection data acquisition and processing 

are described by Foster and others (1999).

The area offshore of Fire Island was resurveyed in May 2011 aboard the motor vessel Scarlett 

Isabella by using an interferometric sonar to acquire bathymetric and backscatter data and a chirp 

seismic-reflection profiler to define the subsurface stratigraphy and structure. The survey area extends 

about 50 km alongshore and about 8 km offshore in water depths ranging from approximately 8 to 32 

meters (m) (fig. 1) and covers approximately 336 square kilometers (km2

). Details of the acquisition and 

processing of these data are described in Schwab, Denny, and Baldwin (2014).

Modern sediment thickness mapped in 2011 covered an area of about 274 km2 (Schwab, Denny, 

and Baldwin, 2014) and was used for comparison to the modern sediment thickness mapped in 1996–97 

(Foster and others, 1999). Ship position for both surveys was determined by using a Differential Global 

Positioning System (DGPS). The layback position of the seismic-reflection system towfish relative to 

the ship was accounted for in both surveys. We estimate that the positions of the seismic-reflection data 

are accurate to within 5 m.

The sediment thickness mapped in 1996–97 by Foster and others (1999) was calculated from 

two-way travel time on seismic-reflection profiles by using an assumed internal velocity of 1,630 meters 

per second (m/s). However, the sediment thickness mapped in 2011 by Schwab, Denny, and Baldwin 

(2014) was calculated from two-way travel time by using an assumed internal velocity of 1,500 m/s. In 

order to compare the 1996–97 and 2011 datasets, the 1996–97 sediment thickness was converted to the 

common internal velocity of 1,500 m/s used by Schwab, Denny, and Baldwin (2014) by multiplying by 

a factor of 0.92. The two datasets were then compared, yielding a difference grid (fig. 3) with a 

resolution of 100-m grid cell size to match the coarser input of the 1996–97 data.

2
A vertical resolution of 50 centimeters (cm) is assumed for sediment volume calculations 

because of a conservative estimate of the vertical resolution limits of the subbottom profiling systems 

used during the 1996–97 and 2011 surveys.

Mapping Results

Previous interpretations of marine geologic mapping data support the hypothesis that the 

Holocene evolution of Fire Island, including its modern decadal to centennial behavior, is linked 

directly to the geologic framework of the inner continental shelf. Schwab, Thieler, Denny, and others 

(2000) and Schwab and others (2013) identified the modern sediment deposit on the inner continental 

shelf offshore of Fire Island as a likely source of sediment required to balance the coastal sediment 

budget, suggesting that an onshore-directed component of the dominant westerly sediment flux (from 

the inner continental shelf to the shoreface), in combination with periodic contribution from beach 

nourishment activities, provides sufficient sediment to maintain island stability west of Watch Hill.

A comparison of modern sediment thickness mapped from seismic-reflection data in 1996–97 

(Foster and others, 1999; Schwab, Thieler, Allen, and others, 2000) and 2011 (Schwab, Denny, and 

Baldwin, 2014) provides evidence to support the hypothesis of shoreward-directed sediment flux. The 

comparison (figs. 3 and 4), which shows changes in sediment thickness greater than 50 cm (the vertical 

resolution limit of the subbottom systems used in the two surveys), illustrates net westerly migration of 

the sand ridges, with erosion on their eastern flanks and crests and deposition on their western flanks, as 

well as significant accretion along the lower shoreface of western Fire Island.

Schwab, Baldwin, Denny, and others (2014) calculated that the modern sediment volume on the 

lower shoreface has accreted 7.8 × 106 cubic meters (m3) and the modern sand deposit on the inner 

continental shelf was reduced by 3.1 × 106 m3 over the 15-year period. This led them to speculate that 

the lower shoreface modern sediment deposit has gained volume at the expense of deflation of the large 

shoreface-attached sand ridges and inner continental shelf in general. This raises a question: if the 

shoreface-attached sand ridges are eroding, how are these prominent features maintained in the long 

term?

While attempting to address this question, an error was discovered in the volumetric calculations 

presented in Schwab, Baldwin, Denny, and others (2014); they did not convert the modern sediment 

thickness isopach of the 1996–97 survey produced by Foster and others (1999) to the common velocity 

of 1,500 m/s prior to comparing to the 2011 data (Schwab, Denny, and Baldwin, 2014). Although 

seemingly a minor correction (see “Data Collection and Processing” section above), following 

conversion the modern sediment volume of the lower shoreface is shown to have increased on the lower 

shoreface by 10.2 × 106 m3 and increased on the inner continental shelf by 10.0 × 106 m3 over the 15-

year timespan; total accretion of 20.2 × 106 m3. This calculation includes the mining of 2.4 × 106 m3 of 

sand for beach nourishment activities (Lentz and others, 2013). Thus, speculation by Schwab, Baldwin, 

Denny, and others (2014) that the lower shoreface has gained volume at the expense of deflation of the 

shoreface-attached sand ridges is incorrect.

Modern sediment thicknesses greater than 50 cm were mapped over 112 km2 in the 1996–97 

surveys (41 percent of the mapped area) and over 119 km2 of the 2011 survey (43 percent of the mapped 

area). The mean change in modern sediment thickness for the mapped area is +0.07 m, with the mean 

gain in areas of accretion +0.56 m and mean loss in areas of erosion -0.14 m. This strongly implies that 

erosion of Pleistocene glaciofluvial and lower Holocene channel-fill deposits exposed at the seafloor 

continues to yield the modern sediment required to balance the coastal sediment budget (Schwab and 

others, 2013) and maintain the shoreface-attached sand ridges.

3
References Cited

Foster, D.S., Swift, B.A., and Schwab, W.C., 1999, Stratigraphic framework maps of the nearshore area 

of southern Long Island from Fire Island to Montauk Point, New York: U.S. Geological Survey 

Open-File Report 99–559, accessed October 2014, at http://pubs.usgs.gov/of/1999/of99-559/.

Leatherman, S.P., and Allen, J.R., 1985, Geomorphic analysis of the south shore barriers of Long 

Island, New York: Boston, Mass., National Park Service Technical Report, 350 p.

Lentz, E.E., Hapke, C.J., Stockdon, H.F., and Hehre, R.E., 2013, Improving understanding of near-term 

barrier island evolution through multi-decadal assessment of morphologic change: Marine Geology, v.

337, p. 125–139.

Schwab, W.C., Allison, M.A., Corso, W., Lotto, L.L., Butman, B., Buchholtz ten Brink, M.R., Denny, 

J.F., Danforth, W.W., and Foster, D.S., 1997, Initial results of high-resolution sea-floor mapping 

offshore of the New York-New Jersey metropolitan area using sidescan sonar: Northeastern Geology 

and Environmental Sciences, v. 19, p. 243–262.

Schwab, W.C., Baldwin, W.E., Denny, J.F., Hapke, C.J., Gayes, P.T., List, J.H., and Warner, J.C., 2014, 

Modification of the Quaternary stratigraphic framework of the inner-continental shelf by Holocene 

marine transgression—An example offshore of Fire Island, New York: Marine Geology, v. 355, p. 

346–360.

Schwab, W.C., Baldwin, W.E., Hapke, C.J., Lentz, E.E., Gayes, P.T., Denny, J.F., List, J.H., and 

Warner, J.C., 2013, Geologic evidence for onshore sediment transport from the inner-continental 

shelf—Fire Island, New York: Journal of Coastal Research, v. 29, no. 3, p. 526–544.

Schwab, W.C., Denny, J.F., and Baldwin, W.E., 2014, Maps showing bathymetry and modern sediment 

thickness on the inner-continental shelf offshore of Fire Island, New York—Pre-hurricane Sandy: 

U.S. Geological Survey Open-File Report 2014–1203, accessed December 2014, at 

http://pubs.usgs.gov/of/2014/1203/.

Schwab, W.C., Thieler, E.R., Allen, J.R., Foster, D.S., Swift, B.A., and Denny, J.F., 2000, Influence of 

inner-continental shelf geologic framework on the evolution and behavior of the barrier-island system 

between Fire Island Inlet and Shinnecock Inlet, Long Island, New York: Journal of Coastal Research, 

v. 16, p. 408–422.

Schwab, W.C., Thieler, E.R., Denny, J.F., Danforth, W.W., and Hill, J.C., 2000, Seafloor sediment

distribution off southern Long Island, New York: U.S. Geological Survey Open-File Report 00–243, 

accessed October 2014, at http://pubs.usgs.gov/of/2000/of00-243/default.htm.

Stone, B.D., and Borns, H.W., 1986, Pleistocene glacial and interglacial stratigraphy of New England, 

Long Island and adjacent Georges Bank and Gulf of Maine, in Sibrava, Vladimir, Bowen, D.Q., and 

Richmond, G.M., eds., Quaternary glaciations in the northern hemisphere: Oxford, Pergamon Press, p. 

38–52.

Williams, S.J., 1976, Geomorphology, shallow subbottom structure and sediments of the Atlantic innercontinental shelf off Long Island, New York: Fort Belvoir, Va., U.S. Army Corps of Engineers 

Technical Paper 76–2, 123 p.

4
5 

Figure 1. Map showing the survey area (shaded in dark gray) offshore of Fire Island, New York, 2011. Inset map 

shows location of study area (outlined in black). Bathymetric contours are in meters (m) below the North American 

Vertical Datum of 1988 (NAVD 88). Figure modified from Schwab and others (2013).
6 

Figure 2. High-resolution chirp seismic-reflection profile and interpretation illustrating stratigraphic features and 

geometries discussed in the text. Location of the profile is shown in figure 3. Approximate water depth in meters is 

based on a two-way travel time of 1,500 meters per second. The Holocene transgressive unconformity is marked 

by a yellow dashed line. Glaciofluvial channels and older Pleistocene sediments are marked by orange dashed 

lines. Figure modified from Schwab and others (2013).
7 

Figure 3. A, Map showing the change in modern sediment thickness greater than 0.5 meter (m) between 

isopachs interpreted from 1996–97 (Foster and others, 1999) and 2011 (Schwab, Denny, and Baldwin, 2014) 

seismic-reflection data, overlain on the 2011 modern sediment thickness isopach (Schwab, Denny, and Baldwin, 

2014), offshore of Fire Island, New York. Blue line shows seaward extent (toe) of the shoreface (Schwab, Baldwin, 

Denny, and others, 2014). Regional bathymetric contours are in meters below the North American Vertical Datum 

of 1988 (NAVD 88). Figure modified from Schwab, Baldwin, Denny, and others (2014). B, Enlargement of area 

shown in A. Change in sediment thickness shown by using a less conservative vertical resolution limit of 10 

centimeters to illustrate net westerly migration of the sand ridges, with erosion on the eastern flanks and crests of 

the ridges and deposition on the western flanks.
8 

Figure 4. Maps showing the 1- and 3-meter (m) contours of sediment thickness from the 1996–97 and 2011 

surveys offshore of Fire Island, New York, of the A, shoreface-attached sand ridges and B, area of accretion on the 

lower shoreface overlain on the 2011 modern sand thickness (Schwab, Denny, and Baldwin, 2014). Yellow lines 

illustrate the southwest migration of contours. See figure 3A for locations. 
Prepared by the Pembroke Publishing Service Center.

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Schwab and others—Maps Showing the Change in Modern Sediment Thickness on the Inner Continental Shelf, Fire Island, New York, Between 1996–97 and 2011—Open-File Report 2014–1238

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20141238